Elliptical Galaxies

M32 - elliptical companion to the Andromeda spiral. Courtesy of SEDS Archive

Introduction

These galaxies have a fairly featureless appearance, and range from objects which are almost spherical, to those which are almost flat. Those which are spherical are classed as E0 galaxies, and those which are flattest are E7, with the types in between being given the numbers E1 to E6. In general, then, an elliptical galaxy is given the label En, where the number n is determined by the galaxy shape. The diagram below shows how this number is calculated.

The problem with this classification is that it is highly dependent on the orientation of the particular galaxy with respect to our viewpoint - that is, it describes how the galaxy appears to us; it does not tell us much about the true physical properties of the object. Unless a galaxy is perfectly spherical (in which case it would be an E0, since the semi-major axis a, and the semi-minor axis b, would be equal as the diagram above shows), it will have a different appearance according to the direction from which we observe it.

Nevertheless, the En system is useful for describing how a galaxy looks in the telescope. The image on the left is of the elliptical galaxy Messier 59 (shown here courtesy of the SEDS Archive). This galaxy is of the E5 class, and the image clearly shows the much more "flattened" appearance, compared with the E0 galaxy Messier 32 (shown at the top of this page). M59 is a member of the Virgo cluster of galaxies.

What determines the shape of elliptical galaxies ?

There is rarely any true structure visible in elliptical galaxies, unlike the magnificent forms we see in the spirals. In fact, elliptical galaxies are vast collections of stars which remain relatively close together because of the gravitational attraction between them. There is no well defined rotation axis, again in contrast to the spirals, so that stars in the galaxy show a wide range of orbital paths around the centre. However, the motions of stars are not entirely random; there is a degree of uniformity in the direction of these motions, and it is this which determines the overall shape of the galaxy.

It is interesting to note that, although the system of classifying the ellipticity of these galaxies illustrated above, is capable of producing labels of up to E10, in reality we only see objects of classes up to and including E7.

The physical nature of elliptical galaxies

The difference between elliptical and spiral galaxies is not a superficial one limited to appearance. They are distinctly different in their content. To begin with, colour photographs of ellipticals show that they are more red in colour than spirals (note, however, that the image of M32 at the top of this page is a false colour one!) The reason for this difference is that elliptical galaxies contain a majority of old stars. In contrast, spirals contain many young, hot blue stars in their discs and in the central regions. Furthermore, elliptical galaxies contain very little gas and dust (a fact which also hampers the formation of new stars).

Elliptical galaxies are found in a much wider range of masses than their spiral counterparts; 100,000 to 10,000,000,000,000 solar masses according to one estimate! Sizes are similarly varied, with observations showing that objects can have diameters of between 1 and 100 kiloparsecs (or 3260 to 326,000 light years). In the light of these facts, it is not surprising to find that elliptical galaxies show a larger range in brightness; some can be up to 10 times brighter than the brightest spirals. At the other end of the scale, the faintest ellipticals can be 1000 times less luminous than the faintest spiral.

Lack of form, an old stellar population - these observations have led astronomers to believe that the elliptical galaxies formed earlier than spirals. This would explain why the stellar population is very old. However, not all of these objects are quite so inactive. Radio astronomers have detected quantities of gas and dust in some ellipticals, and some of these galaxies have been found to be very "noisy" in the radio region of the electromagnetic spectrum.

The most spectacular example of such an active elliptical galaxy is NGC 5128, otherwise known as Centaurus A, and shown above (courtesy of the AAO). in fact, this is not really a pure elliptical galaxy. The dark band running through the centre of the image is a belt of dust, and it appears that this band of dust actually forms a plane running through the galaxy, somewhat reminiscent of the form of spiral galaxies. It is possible that this object is the result of a spiral galaxy being swallowed up by a much larger elliptical object, although this is by no means a certainty. But, whatever the cause, nowhere else in the observable night sky is there another elliptical galaxy quite as strange as Centaurus A.

The image on the right (courtesy of the AAO) shows the giant elliptical galaxy M87 (type E1), along with some smaller companion galaxies. Compare the shape of this object to that of Messier 59, shown above; M87 is very obviously more spherical, hence its classification. This galaxy is found in the constellation of Virgo, and is the largest galaxy in a huge collection of galaxies called the Virgo Cluster. The galaxy, and its retinue of companions, is around 60 million light years distant - which means that we see the galaxy as it actually appeared 60 million years ago.

The galaxy is estimated to contain the same amount of material as would be found in over two and a half trillion Suns. Whilst being an excellent example of an elliptical galaxy, this is also a very unusual object. In 1918 astronomers discovered a jet of material thousands of light years long comming from the centre of the galaxy, almost certainly material which has been ejected from the core of the galaxy. In addition, astronomers studying the radio "noise" from M87 found that the galaxy was extremely energetic. Subsequently, observations using the Hubble Space Telescope have apparently shown that the centre of the galaxy contains a massive black hole, as massive as three billion suns.

A black hole in M32

Recent work performed using theoretical simulations, and observations from the Hubble space telescope, have suggested the possibility of the existence of a black hole in the centre of the galaxy M32 (pictured above). The Hubble Space Telescope (HST) observed the very centre of this galaxy, and was able to measure the motion of stars in the area. Using these measurements, and one of the worlds most powerful computers, astronomers were able to show that these stars behaved exactly as expected if there was a black hole in the galactic centre.

Earlier, in 1992, the HST observed a bright condensation in the galaxy, which was also a hint that a supermassive object existed in the centre.